The invention concerns a magnet system for a nuclear spin tomograph with at least one field producing magnet coil S.sub.1 located in a middle plane and having a radius r.sub.s1 which produces a homogeneous magnetic field along a z-axis directed perpendicular to the middle plane.
A magnet system of this kind is known in the art from EP 0 160 350 B1.
In this publication slice-shaped configurations of magnet coils are described for the production of a homogeneous magnetic field for NMR tomography. These slice-shaped coils comprise ring-shaped concentrically nested partial coils which have currents flowing through them in opposite radially alternating directions. In this fashion these types of coils can be utilized to produce an NMR magnetic field with the necessary homogeneity in the vicinity of the center of the investigational volume.
An advantage of this type of slice-shaped magnetic coils is that very good access to the investigational volume axially as well as diagonally from the side or from above is facilitated. In this fashion, for example, an examining physician can treat portions of the body of a patient located in the investigational volume during an examination of the patient. In particular the possibility is thereby provided for carrying out minimally invasive surgery, whereby the treating physician can utilize NMR images during the operation for at least partial orientation assistance inside the body of the patient. In an ideal case monitoring of the operation is possible using the produced NMR images.
A similarly structured slice-shaped magnetic coil configuration for field production in NMR tomography is known in the art from WO 90/05369. This configuration likewise contains concentric individual coils arranged in the middle plane having differing radii, whereby pole plates made from ferromagnetic material are provided for at axial separations from the slice-shaped coil configuration symmetrically with respect to the middle plane which are attached to an iron housing surrounding the slice-shaped configuration so that a largely closed magnetic flux of the magnetic field produced in the slice-shaped coil configuration is achieved. The pole plates are intended to largely balance out the forces in the wall due to the magnetic flux.
In a configuration of this type it is not, however, possible to achieve a homogeneity volume in the center of the slice-shaped coil configuration, rather two volume regions of high magnetic field homogeneity are produced on each side of the middle plane of the configuration at an axial separation before or behind the slice. Due to the metal plates which block access in the axial direction in this known H-magnet configuration this system offers neither good access in the axial direction nor diagonally from the side or from above to the geometrical center of the configuration.
The magnet system known in the art from U.S. Pat. No. 5,117,188 which also exhibits a slice-shaped coil configuration also fails to offer double-sided axial or sideward diagonal access to the center of the configuration which, in this case, also contains the volume of greatest homogeneity. In this configuration in which the z-axis runs vertically, an asymmetric ferromagnetic plate is provided for below the ring-shaped coil having an axial separation from the middle plane of the coil configuration upon which, for example, a patient being examined can lie. With the assistance of the asymmetrically shaped ferromagnetic plate, the magnetic field within the investigational volume is strengthened and rendered locally homogeneous.
Although the above mentioned EP 0 160 350 B1 describes a slice-shaped magnet coil configuration with which the axial and radially diagonal access to the investigational volume is relatively large, the magnet system known in the art exhibits, however, the disadvantage that, due to the alternating current directions of the partial coils of the slice-shaped configuration, the magnetic field strengths produced in the investigational volume are relatively low despite higher individual currents, since the produced partial fields largely cancel each other. In order to achieve a sufficiently high magnetic field strength with likewise high field homogeneity in the investigational region, it is necessary for these types of systems to be relatively large. In addition, with resistive systems, a very high electrical power for the magnetic field construction is required which leads to a heating of the coils or to cooling problems at high field strengths. Finally, the space requirements for a system of this type are relatively high.
It is therefore the object of the present invention to present a magnet system of the above mentioned kind which, on the one hand, facilitates good access to the investigational volume in the axial direction from the front and at least diagonally from the side and which, on the other hand, can be constructed in a relatively compact fashion for comparable produced magnetic field strengths or which produces a magnetic field with substantially higher field strength compared to magnetic systems with comparable ampere winding numbers and field homogeneity in a comparably large investigational volume.